The American International Clarinet Festival continued last night at Catholic University with Stanley Drucker and the Festival String Quartet in works of Mozart and Brahms. It was a happy coupling, and the soft edges of Drucker's clarinet tones often had the gentle beauty of colors melting in a rainbow.

If Mozart's Quintet for Clarinet and Strings centers on the variation form, Brahms' Clarinet Quintet leads up to its final variations with the logic and spontaneity of an enjoyable romance. This is music of well-bred passion, at once elegant, and red hot. When the 60-year-old Brahms had all but resolved to give up composing, he found inspiration in young Richard Muehlfeld, whom he privately called "Fraulein Clarinet" because of the delicate virtuosity of his playing. For him Brahms created a chamber work of profound feeling and symphonic proportions.

Spontaneity on stage is the result of its opposite, long hours of rehearsal and trust. Accompanying Drucker, the Festival Quartet sounded anything but spontaneous despite some lovely individual contributions. A penchant for underpitching was particularly jarring in the first movement of the Brahms quintet, where the impeccable structure of the score was often veiled in uninvited tonalities. There was also an impatience in the phrasing if not in the tempo, which stuttered against Drucker's careful lines. Still, the playfulness of the third movement was real, and on the whole these glimpses of a difficult score were a fine festival gift.